Polishing article



Patented Nov. 2, 1937 v UNlTED STATES POLISHING narrow Charles E. Wooddeli, Niagara Falls, N. Y., assignor, by mesne. assignments, to The Cal-bonindum Company, Niagara Falls, N. Y., a corporation of Delaware No Drawing. Application March 19, 1934, Serial In Germany November 2, 1932 6 Claims. (Cl. 51-280) This invention relates to novel polishing articles and methods of making the same. More particularly it relates to a polishing wheel of novel composition and structure .and methods of making the same.

In preparing structures which are to possess a high polish in use or are to act as bearing surfaces it is customary to grind the surface successively with abrasive wheels of progressively finer grit. This procedure is followed in order to remove relatively large amounts of stock with the coarser grits and then reduce the irregularities of the surface which result from the deeper cuts of the coarse grits by taking lighter cuts with the finer grits. These finer grit wheels will then remove the "high spots left by the coarse grits and produce an article with relatively shallow cuts or scratches. The limit to the fineness of surface secured with this practise is determined by the coarseness and hardness of the abrasive used in the last wheel applied.

While it is possible by this last mentioned process to make a surface which is apparently very smooth, such a surface will be seen, when examined under a microscope, to consist of a, series of hills and valley This irregularity of surface is objectionable in parts subjected to wear because bearing is made only on the hills and the surface area of these hills" is small relative to the total area of the surface. The wear is consequently increased and the life of the article materially reduced by this condition.

Surfaces smoother than those previously obtainable by grinding with a bonded abrasive article have heretofore been made by a lapping operation because those skilled in the art have heretofore been unable to produce bonded polishing articles which will produce the desired smooth finish. The lapping operation consists in applying a lapping compound to the surface being-finished by means of a so-called lap which is simply an article made of relatively soft and slightly plastic material to approximately fit into the shape of the surface desired in the finished article. Lapping compounds consist, essentially, of a very fine and comparatively soft abrasive material suspended in a suitable carrier. For example such substances as precipitated iron oxide (crocus or rouge), chromic oxide and similar materials are commonly applied by means of a lap which may be of wood, lead or similar soft, yet semi-rigid, character. Polishing is sometimes done by means of abrasive or polishing paper or cloth made of suitable materials suchas crocus fastened to a suitable backing such as paper or cloth by means of an adhesive such as glue.

Polishing by means of lapping operations or polishing paper or cloth produces a very smooth surface and a fineness of finish not previously obtainable by any other known method. This fineness of finish is thought to be due to two causes, namely, the relatively soft and fine abrasive article used and the wiping action obtained by the manner in which the abrasive material is used. The wiping action results from the fact that there is an area of contact between the abrasive article or material and the work in contrast to a line contact between a bonded abrasive wheel and the work.

These methods of polishing however are open to several objections. They are comparatively costly. If done by hand they require a skilled operator, and even then it is practically impossible to maintain the trueness of surface left by the grinding operation; for example, an article ground to be truly round will be finished slightly out of round. If done by machine, these methods require a relatively costly machine which is useless for anything else except polishing and which required special fixtures for each different shape or size finished in the machine.

The present invention contemplates the production of a polishing article by the use of which one can obtain the fineness of finish heretofore obtainable only by means of a lapping operation with the accuracy of finish heretofore obtainable only by means of a rigidly bonded abrasive article.

According to the present invention polishing articles, specifically wheels having the same general form and outward structure as abrasive wheels, are produced by forming polishing powders such as those heretofore used only in lapping operations lnto suitable articles by means of suitable bonding material. For example, the present invention contemplates the formation of wheels consisting of precipitated aluminum oxide powder held in the proper shape by a resin bond.

This invention also provides a method whereby polishing articles having new physical characteristics may be produced. For example, polishing articles produced according to the present invention have a relatively low apparent density combined with a relatively high modulus of elasticity combined ,with a high unit resilience. A polishing wheel produced according to this invention has a fineness of particle size and a softness never before formed into a bonded article and the article itself possesses suillcient resilience to allow the wheel to flatten slightly upon contact with the work and thereby apply the wiping action described above in the discussion of the polishing operation.

Granulated fused alumina, silicon carbide and other abrasive grain have heretofore been formed into abrasive articles by means of resin bonds. For example phenol, formaldehyde and abrasive grain have been mixed together and the mixture treated with heat to cure the resin formed.

By another previous method powdered resin and abrasive grain have been intimately mixed and the mixture wetted with a small proportion of a solvent such as furfural and the resulting sticky mass pressed into shape and cured.

By another previous method plaster of Paris, shellac, and abrasive grain have been mixed together and the mixture heated to melt the shellac and subsequently heated to cure it.

By still another previous method the abrasive grain to be used has been heated and a phenolic resin and plaster of Paris added to it. After thorough mixing the mass was cooled to harden the resin and granulated to a size larger than that of the individual abrasive grain. The granulated resin-abrasive grain mixture was then pressed into proper shape and heated to cure the resin.

None of the methods previously used in forming abrasive articles however, has been found suitable for forming the new polishing articles of the present invention. The methods previously employed have all involved what may be called a set mix in contradistinction to the mix used in the method of the present invention.

In known methods of making abrasive articles the resin bond was used mixed with the abrasive grain in a naturally liquid state or was treated with a solvent after admixture with the abrasive grain, or was heated to melt it prior to said admixture or was added to abrasive grain which was itself heated. With such methods the resin bond did not retain a granular condition but flowed around the abrasive grains to form more or less a continuous film around the grains. When an abrasive article was made from said mixture, it was found that the article comprised a rigid continuous frame work of bond material, thinly spread out, holding the abrasive grains rigidly within its structure and contacting with them at practically all points. This rigidity of structure was a beneficial characteristic of abrasive articles and these methods were therefore satisfactory for the production of such articles.

These methods, however, are not satisfactory to produce the new polishing article of the present invention. When one attempts to make the polishing article of the present invention according to certain prior methods, it is found that the resulting article is extremely non-homogeneous. Some portions of the article will be extremely hard and dense, apparently containing a large proportion of bonding material, while other portions will be very porous and structurally weak. Furthermore the article as a whole will be relatively dense and have a low modulus of rupture, a high modulus of elasticity and a low unit resilience. If the method is varied inaccordance with other prior methods to improve the modulus of rupture and the resilience, it will be found that the density is still higher and the modulus of elasticity is extremely high.

In connection with the present invention it has been found that the lack of homogeneity resulting when one attempts to produce polishing articles of the present invention by the methods employed previously in making abrasive articles is due to the extreme fineness of the polishing powders used in the present invention. When the fine polishing powders are mixed with binder and formed into articles by the methods previously known the resulting article is not strong enough for practical purposes. Furthermore, as described above in connection with the manufacture of abrasive articles, the wet resin bonds tend to run together to form a continuous rigid structure holding the individual particles of powder rigidly in the structure with the result that articles so produced in addition to other deficiencies are rigid and non-resilient.

The present invention provides a new method of producing a polishing article of the present invention whereby the article has the desirable characteristics set forth above. According to this method polishing powders of extreme fineness are mixed in appropriate proportions with a finely powdered or pulverized solid resin product. The two constituents are thoroughly mixed and the mixture is then put in a suitable mold, such as is commonly used for forming abrasive articles, and pressed under high pressure, for example, 2000 pounds per square inch. While under pressure the plunger of the mold may be clamped in place by any convenient means and the clamped mold removed from the press and placed in an oven, the temperature of which may be about 350 F. The molded article may be cured at this temperature for a period of from twelve to sixteen hours, after which it may be removed from the mold and. finished by machining in the manner heretofore used in finishing abrasive articles. By a modification of this method the article may be pressed at the high pressure, e. g. 2000 pounds per square inch, removed from the press and cured in an oven under a pressure of the order of 20 to 50 pounds per square inch maintained by placing a suitable weight on the plunger of the mold. The article may also be cured in a socalled hot press, but if this method is used the pressure should be reduced after the initial press to a figure of the order of magnitude given above. By still another modification the article may be pressed under the initial high pressure, heated I under reduced pressure to soften the resin grains slightly, cooled, removed from the mold and cured by suitable means.

The articles produced by this method comprise a homogeneous, uniform mixture of polishing powder and resin bond. The particles or flakes of powder are bonded together by means of discrete particles of bond which apparently do not substantially lose their granular condition during the process. The mixture is apparently bonded together by a point to point contact of the particles. Instead of flowing around the particles of polishing powder and substantially enveloping the individual particles thereof whereby the particles of powder are stuck to each other by means of thin films of bonding material, the granulesor particles of bond apparently retain their shape substantially and each particle of bond apparently contacts with a plurality of particles of polishing powder at a plurality of points on its surface, and each particle of polishing powder apparently contacts with a plurality of particles of bonding material at a plurality of points on its surface. The structure produced by this means results in an article of low apparent density and high resilience, having a low modulus of elasticity, at the same time having a high modulus of rupture. For example, an article made by this method, of

precipitated aluminum, oxide powder and bond may have the following physical characteristics.

Modulus of rupture 5000 Modulus of elasticity 2.0 10 Unit resilience .70 Apparent density 1.92

The apparent density of articles made by this method may be as low as 1.6. This property is mentioned merely as a means of furnishing identification of articles made by this method and is not intended as a limiting characteristic. Apparent density will vary with the kind of polishing powder used, the pressure applied in the forming and curing steps and other variables.

The article of this invention may be made from any of the polishing powders previously used in lapping operations or their equivalents, for example, precipitated aluminum oxide powder, green chromic oxide or an iron oxide such as rouge or crocus. The three oxides specifically mentioned are valuable for polishing primarily for two reasons, namely, their extremely fine particle size and their hardness. They are sumciently hard when rubbed over metals to remove small amounts of metal, but are so fine that the depth of cut taken is insufficient to produce scratches which are visible. They therefore produce very smooth surfaces and mirror like finishes. The bond used may be any suitable resin which can be pulverized at ordinary temperatures to produce a dry powder such as dry powdered phenolic resin or dry powdered alkyd resin. I may use various proportions of powder to bond; and, in fact, in order to keep the grade comparable when the other oxides are used, it is necessary to change the proportions approximately in the ratio of the densities of the polishing powders. I have also found that grade can be altered by a change in the pressure at which the mold is clamped. The fineness of the bond should approximate that of the polishing powder. As a practical matter in such fine powders as used herein the size of the particles may vary to a relative degree which would not be permissible in a grit size of abrasive grain. Furthermore the fine particles tend to form aggregates which, while larger than the average individual particles, are of the same order of size in the tolerance permissible in such fine powders. These aggregates may be broken up temporarily but they tend to retain their aggregate form and function as particles and are herein included in that expression. Due to their soft nature they break up readily in polishing operations without scratching. The average size of the powder and bond particles may vary with respect to each other in the same wide limits, but it may be pref erable to have the bond of an average size equal to or smaller than that of the powder.

A description of a method of making a polishing wheel according to the present invention will now be given. It is to be understood however that the example is illustrative only and not restrictive.

930 grams of precipitated aluminum oxide of such fineness that 80% of the material will be of the size equivalent to a 600 mesh screen are mixed with 70 grams of a solid phenolic condensation product resin such as that known to the trade as Redmanol (IR-0214 which has been pulverized to such a fineness that it is substantially as fine as the polishing powders and will pass through a 150 mesh screen. This material is a mixture containing phenol condensation products in the A .hardened to the infusible C stage by heating.

The materials are thoroughly mixed, for example by rolling in a tumbling barrel or a ball mill, and are then screened through a 100 mesh screen. these two operations being repeated to ensure a uniform distribution of the resin through the powder. The mixture is then put into a suitable mold and cured as described previously.

The precipitated aluminum oxide cited in the example has properties which are distinctly different from those of the crystalline variety which is widely used as an abradant. It is, for example of different crystalline form. It may be prepared by precipitation from a solution of the crude alumina ore in caustic .soda followed by heating to a little over 500 C. The heating drives ofi the water of crystallization carried down when alumina is precipitated. The product Just mendex of refraction which is 1.69 for gamma alumina and 1.76 for alpha alumina. Upon heating from 500 C. to 1100 C. the index apparently experiences a gradual increase as crystals with indices between 1.69 and 1.76 have been found. The term unfused trivalent oxide is used herein to refer to any of these oxides in the state formed by precipitation followed by heating at temperatures below the fusing point of the oxide. though the applicant has found thatin most cases precipitated aluminum oxide makes the most satisfactory polishing article, in some instances green chromic oxide or rouge or crocus appear to be more suitable and the process applies equally well to the use of these materials and any other powders of extreme fineness and uniformity whereby they will function as a polishing powder, and produce fine finishes without undue scratching.

'It will thus be seen that the present invention provides a new polishing article and a method for making the same whereby polishing operations may be carried out to produce the fineness of finish usually or heretofore obtained only by the use of lapping or similar polishing operations and the accuracy of surface heretofore obtained only by the use of abrasive articles. The present invention provides a simple and efficient method of making the polishing article and the article itself has a low modulus of elasticity and a high resilience coupled with sufiicient structural strength.

It has been observed that when a wheel made in accordance with the present invention is placed on a suitable machine such as is ordinarily used for fine grinding of the so-called precision type and put into contact with a piece of material which is to be polished, the feed required to produce a given depth of cut is appreciably greater than is needed with the ordinary abrasive wheel. This fact indicates that, due to its resilience, the new polishing article tends to compress under stress of polishing by means of which the wiping action ordinarily obtained by a lapping operation can be obtained with a relatively rigid bonded polishing article. The use of wheels made according to the present invention requires no special machinery as the operation is performed in the usual grinding machine and is merely an additionaloperation or may be even merely the replacement of the final grinding operation as usually performed. In using these wheels they are mounted on suitable machines and the same operation is performed as is ordinarily done in fine grinding except that the wheel is fed further for any given depth of out than is ordinarily .required or possible. The machine should be in the condition always required for fine grinding, that is, it should be sufiiciently rigid and in the state of repair such that there is no lost motion or play in any of the moving parts. The requirements for using wheels made according to the present invention, however, are no more rigid than those of any fine grinding operation.

I claim:

1. A method of making a polishing article which comprises forming a dry mix consisting essentially of an intimate mixture of finely divided polishing powder and a finely powdered normally solid fusible resin and compacting the mixture under pressure and without application of heat throughout the mixture, reducing the pressure on the compacted mixture to not more than that required to maintain the desired shape, heating the mixture under the reduced pressure to soften the resin particles to cause them to adhere to the adjacent particles of powder and to agglomerate and harden the mixture.

2. A method of making a polishing article which comprises forming a dry mix consisting essentially of an intimate mixture of finely divided polishing powder and a finely powdered normally solid fusible resin and compacting the mixture under pressure and without application of heat throughout the mixture, reducing the pressure on the compacted mixture to not more than pounds per square. inch to maintain the desired shape, heating the mixture under the reduced pressure to soften the resin particles to cause them to adhere to the adjacent particles of powder, and to agglomerate and harden the mixture.

-3. Amethod of making a polishing article which comprises forming a dry mix consisting essentially of an intimate mixture of finely divided polishing powder and finely powdered normally solid fusible resin and compacting the mixture under 2000 lbs. pressure without application of heat throughout the mixture, reducing the pressure on the compacted mixture to not more than that required to maintain the desired shape, heating the mixture under the reduced pressure to soften the resin particles to cause them to adhere to the adjacent particles of powder, and to agglomerate and harden the mixture.

4. A polishing article produced by forming a dry, intimate mixture of finely divided polishing powder and finely divided fusible resin, compacting the said mixture under pressure and without application of heat throughout the mixture, reducing the pressure on the compacted mixture to not more than that required to maintain the desired shape and heating the mixture under the reduced pressure to soften the resin and to agglomerate and harden the mixture.

5. A polishing article produced by forming a dry, intimate mixture of finely divided polishing powder and finely divided fusible resin, compacting the said mixture under pressure and without application of heat throughout the mixture, reducing the pressure on the compacted mixture to approximately 50 pounds per square inch and heating the mixture under the reduced pressure to soften the resin and to agglomerate and harden the mixture.

6. A polishing article produced by forming a dry, intimate mixture of finely divided polishing powder and finely divided fusible resin, compacting the said mixture under a pressure of approximately 2000 pounds per square inch and without application of heat throughout the mixture, reducing the pressure on the compacted mixture to approximately 50 pounds per square inch and heating the mixture under the reduced pressure to soften the resin and to aggl'omerate and harden the mixture.

CHARLES E. WOODDELL. 

